added invalidity hints to item side-properties

- added :invalid definition to side-properties CSS
-  defined common color for invalidity
- now using disabled input for automatically calculated properties

src/scss/components/_description.scss

@@ -22,7 +22,12 @@
             text-align: left;
             width: calc(100% - 2px);
         }
-
+        input:invalid {
+            background-color: $c-invalid-input;
+        }
+        input:disabled {
+            background-color: transparent;
+        }
         input[type="checkbox"] {
             width: auto;
             height: 100%;